In this article, we have systematically studied the effect of molecular weight on poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(5,5'-thienyl-4,4'-dihexyl-2,2'bithiazole)-2,6-diyl], a novel cathodically coloring electrochromic polymer. The polymer with higher molecular weight can stack much better due to the stronger pi-pi* interaction and was found to exhibit better charge transfer property and lower internal resistance. These results have shown to enhance the coloration efficiency of the polymer remarkably. For the molecular weight of 33.6 kg/mol, the coloration efficiency is 423 cm(2)/C, much higher compared with the low molecular weight polymer (6.3 kg/mol, 226 cm(2)/C). (C) 2011 Elsevier B.V. All rights reserved.